1. Field of the Invention
The present invention relates to a cathode for an electron tube and a method for manufacturing the same, and more particularly, to reduction of a Moire phenomenon and improvement of the lifespan characteristics of oxide-coated thermoelectron emission electrodes (oxide electrodes) widely used in general Braun tubes.
2. Description of the Related Art
A conventional cathode for an electron tube is illustrated by a sectional view in FIG. 1. As shown in FIG. 1, the cathode includes a disc-type metal substrate 12, a cylindrical sleeve 13 which supports the metal substrate 12 below the same and installs a heater 14 acting as a heater of the cathode, and an electron emissive material layer 11 attached to the top of the metal substrate 12. Such an oxide cathode for the electron tube can work at a low temperature of 700–800° C. (Celsius) due to its low work function and has been widely used in Braun tubes.
A conventional oxide cathode is manufactured by forming the electron emissive material layer 11 on the metal substrate 12, which contains nickel (Ni) as a major component and traces of silicon (Si), magnesium (Mg), tungsten (W), etc., as a reducing agent. The electron emissive material layer 11 is formed by spraying a suspension of carbonate on the metal substrate 12. The carbonate suspension is prepared by dissolving a binder such as nitrocellulose in an organic solvent and mixing the solution with an alkaline earth metal carbonate containing barium as a major component, preferably, a ternary carbonate of (Ba,Sr, Ca)CO3 or a binary carbonate of (Ba,Sr)CO3. The carbonate is converted into oxide during the evacuation or activation process in the manufacture of the cathode to act as the electron emissive material layer 11.
The principles of operation and electron emission of the oxide cathode are as follows.
A carbonate mixture is coated on a metal substrate by spraying or deposition, as described above, the resulting cathode is mounted into an electron gun to assemble an electron tube. During evacuation of the electron tube, the carbonate is heated by a heater to about 1000° C., and barium carbonate is converted into barium oxide:BaCO3→BaO+CO2↑  (1)
During the operation of the cathode, the resulting barium oxide is reduced through a reaction with a reducing agent such as Si or Mg at the boundary of the metal substrate and produces free barium that triggers electron emission:BaO+Mg→MgO+Ba  (2)BaO+Si→Ba2SiO4+2Ba  (3)
Free barium acts as an electron donor, so the oxide cathode becomes an n-type semiconductor during the cathode operation. In general, when a large quantity of current flows through a semiconductor, the semiconductor generates Joule heat by its own resistance. If the generation of the Joule heat continues for a long time, material evaporation or melting occurs due to the heating of the cathode, which causes degradation of the cathode. In other words, when existing oxide cathodes are used under a high current density applied to enhance the electron emission density of the cathode, there is a problem of the degradation of lifespan characteristics due to the generation of the Joule heat.
As is apparent from the reaction schemes (2) and (3) above, byproducts such as MgO and Ba2SiO4 as well as free barium are produced. Those byproducts accumulate to form an intermediate layer between the electron emissive material layer and the metal substrate. The intermediate layer acts as a barrier to hinder the diffusion of the reducing agent such as Mg or Si. As a result, generation of free barium that contributes to the emission of electrons becomes difficult, which is another cause of the cathode's lifespan reduction.
To address the problems described above, an approach of providing conductivity by mixing carbonate with grounded conductive materials has been made to suppress the generation of the Joule heat. However, the addition of conductive materials sufficient to provide a desired conductivity lowers electron emission characteristics.
As an example, EP 0685868 discloses a cathode for an electron tube, so-called “hot isostatic press (HIP) cathode”. An electron emission material layer of the cathode is manufactured by mixing grounded metal nickel with carbonate and molding with the mixture under high-temperature and high-pressure conditions. Since the electron emission material layer containing a large amount of metal nickel has conductivity, like a metal, during the operation of the cathode, the electron emission material layer has a longer lifespan under a load of high current density. Disadvantageously, the cathode has a high working temperature of 850° C., which is 50° C. or greater higher than conventional oxide cathodes, and is manufactured by the complicated process described above at higher cost.
EP 0560436 B1 discloses the formation of a conductive path following the principle of percolation by the addition of a globular metal of 20–80% by volume to the electron emission material layer of a conventional oxide cathode to improve the lifespan characteristics of the cathode oxide. For a desired percolation effect, at least 30% by weight globular metal needs to be, incorporated into the electron emission metal layer, which reduces a relative amount of the electron emission material in the layer, thereby lowering the initial emission characteristics of the cathode.
S. N. B. Hodgson et al. discloses an oxide cathode with a percolation path formed by the addition of needle-shaped nickel particles of 2.5–5% by volume to the electron emission material layer in an article entitled “Progress on the Percolation Cathode” (IDW '99 Proceedings of the Sixth International Display Workshops CRT 6-4 (Late-News Paper)). The electron emission material layer of the oxide cathode is formed by a general spraying method, so there is a problem of clogging of spray gun nozzles by metal particles and a great surface roughness of the cathode.
Spraying methods of coating a suspension of carbonate particles by spraying through a spray gun are relatively easily applied to the manufacture of the cathode but are limited in forming a uniform, dense, coated layer because air pressure is the only force applied to coat the layer. In particular, the structure of an electron emission material layer coated by the spraying method is shown in FIGS. 2 and 3. FIG. 2 is a 400×-magnified electromicroscopic photograph of the electron emission material layer coated by the spraying method, where non-uniform particle-to-particle voids, a great surface roughness, and a sparse structure are apparent. FIG. 3 is a 3000×-magnified photograph of the surface of the electron emission material layer of FIG. 2, which confirms non-uniform particle and pore sizes in the electron emission material layer.
As described above, the surface of the carbonate coated by the spray method is uneven, has the non-uniform K-phase distribution, and increases the consumption of the carbonate. Also, the carbonate layer has poor COEK characteristics with a deviation in thickness of ±10 μm (microns). These results cause a Moire phenomenon and thus degrade picture quality.
The Moire phenomenon is defined as non-uniform luminance distribution due to a localized difference in electron emission quantity reaching the surface of a Braun tube due to nonuniform electron emissions during the cathode operation, which occurs when the cathode surface is rough, thus resulting an interference pattern between electron beams and screen dots. The Moire phenomenon is known to be more serious with increasing cathode surface roughness.
To prevent this Moire phenomenon, a method of manufacturing a cathode by screen printing has been suggested. The screen printing method makes the cathode surface uniform, and the quality degradation due to the Moire phenomenon is improved. However, an increase in the density of the carbonate layer causes sintering of the cathode to occur and adversely affects on the cathode lifespan.
In particular, when a cathode ray tube (CRT) is operated for a longer period of time, sintering of the cathode occurs. If the cathode has a non-uniform structure, initially formed pores rupture and shrink to increase the distance between the cathode and a G1 electrode. As a result, a potential difference between the cathode and G1 electrode, which is predetermined for the control of emission electron beams, changes, and a reduction in the quality of charge emission degrades the lifespan and luminance of the cathode.
If a cathode with the electron emission material layer having a non-uniform particle size, pore size, and surface smoothness, as described above, is inserted into an electron gun, product quality and reliability degradation may result. The cathodes described in the references described above still have that problem.
Recently, the tendency for a Braun tube for television receivers or monitors to have high luminance to meet high-definition and large-sized screen requirement has increased a need for a high-current density, long-lifespan cathode, which could not be realized with conventional oxide cathodes having the problems described above.
An impregnated cathode has been known to have long lifespan under a load of high current density. However, it is manufactured by complicated processes and has a working temperature of 1000° C., which is higher than oxide cathodes, so there is a need to form electrode parts of the electron gun using high-melting point materials. This increases the manufacturing cost and thus is impractical. In a practical aspect, it is most economical to improve existing cheap oxide electrodes to have a high current density and extended lifespan.